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Q&A forum: Centrifugal and Axial Fan Calculator

All relevant questions concerning this program will be posted here along with our answers for everyone to view.
Please forward your questions to info@calqlata.com or visit our Contact Us page.

Is it correct to say that if the discharge were completely blocked off that the static pressure would be equal to the calculation of the pressure increase across the impeller??

If you blocked off the exit you would achieve maximum pressure, but it would not be that defined by the Fans Calculator.
The Fans calculator uses Charles Innes’ theory which relies on the flow of air across the inlet tip of the blades.
If you have no flow, the theory doesn’t apply.

A common problem with this calculator appears to be the use of the gas constant (Ra) in Imperial units
We have recently had clarification on this issue that may be of assistance:

Your client appears to be using lb, ft & R Imperial units in his calculation, together with an input value for Ra of 0.07666666

Ri = 8.24992342 J/K/mol
RAM = 0.029324 kg/mol {air}
Ra = Ri/RAM = 281.336905617022 J/kg

J > ft.lb = 0.73756215
K > R = 1.8
kg > lb = 2.20462262

Ri = 3.3804618085 ft.lb/R/mole {Ri x 0.73756215 ÷ 1.8}
RAM = 0.064648354 lb/mole {RAM x 2.20462262}
Ra = Ri/RAM = 52.28999061 ft.lb/lb

I would normally expect therefore, that his input value for Ra should be 52.28999061 for air

I have purchased your Fan calculator and am puzzled about something.

We wish to design a fan that we cannot buy. We are looking for 0.25 PSI outlet pressure and a minimum flow rate of 1.5 SCFM. We expect to have to use more than one stage on a centrifugal style fan.

In order to reduce the number of stages we think we should use the backward facing blade as that gives us the greatest pressure increase.

But, when we adjust the inlet and outlet angles the calculator suggests the highest pressures are at inlet angle of small like 10 and outlet angle big like 90. That doesn’t make sense with your chart saying that backward facing blades give the highest pressures. To get a backward facing blade, I would expect to use a small outlet angle to increase pressure, not a large one. We look at the “pressure increase across the impeller” output as we adjust the angles.

What are we doing wrong??

The problem is the requirement for a high outlet pressure relative to the desired flow rate.

A high (reversed) inlet angle (θᵢ) will artificially increase pressure for best results.

You can play with the input and output diameters and the impeller width to achieve the desired flow rate.

You should remember, however, that the Fans calculator only provides the performance characteristics of the impeller.
Alternatively, You could design an impeller that gives a lower pressure and higher flow-rate and then increase pressure and decrease flow rate by playing with the casing outlet dimensions (PV=RT).

We should point out that a casing outlet with cross-sectional area no smaller than the impeller outlet area will result in the lowest noise-level.

I want build centrifical fans. Can your calculator do the design if I give static pressure and volume plus suggest some sizes?

Our Fans calculator calculates the airflow and power consumption for an impeller.
You would have to design the casing/cowling yourself and calculate the effects that would have on the impeller performance.
The casing/cowling options are infinite and do not lend themselves to general calculations.

We have had one customer complaining that the fan calculator doesn't work.

But on sight of his input data his blade angles are completely incorrect, and he refuses to accept this fact.

We have therefore decided to provide a general response to ensure that future customers with no experience or knowledge of the subject are aware of the parameters required for an impeller before trying to design one.

Fan technology is well established and proven to work for all blade angles that comply with Charles Innes' theory, which has been the industry standard since 1916
Impeller design is difficult to understand if you don't take the time to try and understand the basic principles.
Performance is as much dependent upon casing design as it is on the impeller. The Fan calculator only calculates the performance of the impeller.
It is difficult to accommodate casing design as the options are infinite. However, the principle design requirement for a casing is the relative areas between impeller outside perimeter and casing outlet. A larger casing outlet will assist flow, a smaller outlet will assist pressure.

The customer concerned set his blade outlet angle such that it is driving air back into the impeller with greater energy than his inlet angles are able to overcome. He has completely misunderstood the basic principles of driving air through an impeller and refuses to accept the fact.

Inlet blade angles greater than 90° will not drive air out through an impeller, they will drive it back into the inlet cavity.

Moreover, if you do not set your inlet blade angles shallow enough to provide sufficient positive outward drive to overcome inward drive from outlet blade angles greater than 90° the theory will become unstable, as would be such an impeller.

It is important you try to understand the behaviour of air as it passes through the fan. Whilst it is largely based on common-sense, if you ignore basic flow characetristics you will never get your impeller to work, theoretically or practically.

Please take look at the tips we provide in our technical help page

The rule of thumb "one impeller volume per revolution" has been called in to question ...

As a result of this question, CalQlata successfully carried out an internal verification based upon the energy required to shift such a mass.

Whilst it was considered prudent to re-issue the 'Fans' calculator now calculating the impeller speed (RPM) required to generate an entered value for volumetric flow rate based upon the required energy;
the aforementioned rule of thumb still applies.

Unfortunately, I am not a fan expert either! The power and pressure seem OK, but the flow rate is the problem.

We have measured directly the flow rate and although there are obviously some errors in measurement, we seem to have broadly similar practical results of less than 1m3/s. What I really would like to know is the accuracy of the 'rule of thumb' (effective rotor volume x rpm), which I cannot seem to corroborate on a google search.

Does the calculator just produce this value regardless of any other input values?

The standard theory on centrifugal fans was generated by Charles H Innes in 1916 ("The Fan"), and it appears to have stood the test of time.
His theory is based upon the aerodynamics of the blade transferring air from the toe of the blade to the outer lip in a single revolution of the impeller.
He also states that the greater the number of blades the more uniform the flow; i.e. more of the airflow will be laminar (less turbulent). An infinite number of blades will generate the most uniform flow. However, he also states that this comes with increased skin friction.
At some point the losses from skin friction will exceed the inlet and outlet losses and a compromise is needed.
Innes suggests that six blades is the best for a low aspect ratio (<0.67) increasing to twelve blades for aspect ratios approaching one.
This recommendation is based upon his own (extensive) experience.

Twelve blades, however, may not be suitable for very large diameter impellers of high aspect ratios.
In my own (less extensive) experience, I have settled for five blades for aspect ratios of 0.5 increasing to an inlet pitch of similar dimension to the radial depth of the blade.
I would multiply the skin friction in the calculation by the number of blades and would also use a higher value than that suggested by Innes for frictional resistance (0.125) on the basis that 'used' fan blades (that have been in service for some time) will have slightly eroded surfaces (i.e. 0.15 to 0.2).
With regard the expected accuracy of the calculator, if Innes is correct, then the calculations in CalQlata's Fans calculator should be ±0%. But I would only consider this to be the case for properly designed impellers in dry air. Moisture in the air (>1%) can cause a reduction in efficiency. In normal everyday conditions, with a used fan that has been properly designed and maintained, I would expect an accuracy better than ±10% (i.e. ±4% to ±8%), but this does not include the effects of the inlet and outlet diffusers, which can improve or reduce the impeller's effective efficiency.

I (personally) have never seen a fan deliver more than its impeller volume per revolution unless the atmospheric outlet pressure is less than the inlet pressure (an effective vacuum). Therefore, I am unable to refute Innes' theory. I am happy, however, to leave this debate open to anybody that can show Innes' theory to be incorrect. Please let us know if you can do this mathematically and supported with practical evidence.

Can you comment on the limitations of the equations that were implemented in this software? Specifically with regards to impeller size. I’m investigating design changes on a relatively small impeller (3-4”), and so far this software is predicting an output flow that is much higher than has been empirically captured. Any feedback would be appreciated.

The theories in the calculator are correct for all sizes, i.e. for fans with impellers smaller than a millimetre to greater than 10m.
The problem with ever decreasing size is friction.

As your fan gets smaller the ratio of surface area with volume increases, and the smaller it gets the greater this ratio becomes.
If we use a pipe as an analogy:
Dia   Area               Volume           A:V        δε
24   150.7964474   1809.557368   0.083
23   144.5132621   1661.902514   0.087   4.167%
22   138.2300768   1520.530844   0.091   4.348%
21   131.9468915   1385.442360   0.095   4.545%
20   125.6637061   1256.637061   0.100   4.762%
19   119.3805208   1134.114948   0.105   5.000%
18   113.0973355   1017.876020   0.110   5.263%
17   106.8141502   907.9202769   0.120   5.556%
16   100.5309649   804.2477193   0.125   5.882%
15   94.24777961   706.8583471   0.13 0  6.250%
14   87.9645943   615.75216010   0.143   6.667%
13   81.68140899   530.9291585   0.154   7.143%
12   75.39822369   452.3893421   0.167   7.692%
11   69.11503838   380.1327111   0.180   8.333%
10   62.83185307   314.1592654   0.200   9.091%
 9   56.54866776   254.4690049   0.220   10.000%
 8   50.26548246   201.0619298   0.250   11.111%
 7   43.98229715   153.9380400   0.286   12.500%
 6   37.69911184   113.0973355   0.330   14.286%
 5   31.41592654   78.53981634   0.400   16.667%
 4   25.13274123   50.26548246   0.500   20.000%
 3   18.84955592   28.27433388   0.670   25.000%
 2   12.56637061   12.56637061   1.000   33.333%
 1   6.283185307   3.141592654   2.000   50.000%

In this case ‘δε' represents the increase in inefficiency over the previous size
As A:V increases the ratio of surface area to volume increases, and this increase is exponential. As you can see from the table when the pipe size falls to 3" the increase becomes rapid. Look at the difference between a 24" to 23" (4%) and that for 4" to 3" (25%)

Surface friction has a far greater effect on efficiency in a fan than it does in a pipe because the ratio of surface area (contact surface) is greater than in a pipe. Therefore, a similar table to that above for fans would show an even more marked increase at smaller diameters.

Centrifugal fans are less suited to smaller diameter for a number of reasons.
1) The surface area in contact with the flowing air is greater than for axial fans
2) The air is forced to change direction
3) The shape of the inlet and outlet ducts is not best suited for reducing frictional losses (circular is better than rectangular)

This is why centrifugal fans tend to be targeted for larger fans and axial configurations for smaller diameters
I am not saying you shouldn't try to design a small centrifugal fan if that best suits your purposes, simply that you take extreme care in its design. For example, the following should be considered:
1) Use a material with a very low surface friction (or coat the material, but bear in mind that the loss of the coating during the design life will result in a loss of efficiency).
2) Reduce the number of blades
3) Minimise the surface area of material in contact with the flowing air
4) Eliminate sudden changes in shape

For what it’s worth, if I were designing a very small high-performance fan, I would start with a multistage axial configuration (along with suitable venturies if I was looking for pressure as opposed to flow).

Follow-Up Email:

I believe that your input/output data is based upon the following units:
Input Data:
θᵢ,   45 {°}
θₒ,   120 {°}
Øᵢ,   0.02 {m}
Øₒ,   0.075 {m}
w,   0.01 {m}
ρᵢ,   1.165 {kg/m³}
pᵢ,   101322.5 {N/m²}
Ṯ,   291 {K}
g,   9.80663139 {m/s²}
Rₐ,   8.3143 {J/K/kg}
F,   0.125
N,   7000 {RPM}
Output Data:
Q,   0.004788 {m³/s} [287.28 l/min]
H,   38.207032 {m}
δp,   436.50484 {N/m²}
pₒ,   101,759.00 {N/m²}
ρₒ,   42.058538 {kg/m³}
ε,   47.55515 {%}
T,   0.002851 {N.m}
P,   4.394551 {N.m/s}
A,   0.001486 {m²}
v,   3.221569 {m/s}
Lˢ,   0.00427 {m}
Lᶠ,   0.035085 {m}
Lᵉ,   42.096188 {m}
vᵢ,   7.625286 {m/s}
vₒ,   28.734015 {m/s}
v₁ᵢ,   7.619792 {m/s}
v₁ₒ,   2.031945 {m/s}
v₂ᵢ,   7.330382 {m/s}
v₂ₒ,   27.488935 {m/s}
v₃ᵢ,   10.776012 {m/s}
v₃ₒ,   2.346287 {m/s}
v₄ᵢ,   -0.289409 {m/s}
v₄ₒ,   28.662079 {m/s}

With regard to your concerns about flow rate:
Q = 0.004788m/s = 287.28 l/min
A very good rule of thumb for any fan is that its impeller will pass its (contained) volume of air in one revolution.
If I apply this argument to the volume of your impeller, I get: 287.2593783 l/min
So the calculator and the rule of thumb both appear to be working correctly.
However, the above would only be achievable if chamber, impeller, inlet and outlet designs added nothing to the losses#. As mentioned before, their influence becomes significantly greater as the size of the fan reduces. Like you, I would expect closer to 100 l/min for a centrifugal pump of the size you are designing unless all aspects of the fan design were perfect in every way. In fact, even with the best possible materials and designs, I would not expect see better than 200 l/min for such a small (centrifugal) fan
With larger fans (i.e. 6" and greater), it should be much easier to achieve the calculated values.

May I venture a couple of comments on the input data?
The value of Rₐ is for the specific gas constant;
Rₐ (Rᵢ/RAM) = 283.5312934 J/K/kg
as opposed to ideal gas constant;
Rᵢ = 8.3143 J/K/mole
You can see how we use these symbols in our definitions page {http://calqlata.com/help_definitions.html > Gas Constant}
The differences you will notice are in outlet density {ρₒ}, the minimum area diffuser requirement {A} and all the velocities {v}

The recommended angle for the blade inlet should be used where possible as you will see improvements in efficiency, outlet pressure, outlet velocity and power consumption. Whilst I agree that such improvements are very small between 45° to 46.109°, every little helps when attempting to minimise defects (for such small fans).

I notice that the outlet angle (120°) has turned the blades from backward facing to forward facing (see http://calqlata.com/productpages/00060-help.html Fig 3). I am not sure if this was intentional (special requirements) but the smaller the outlet angle for a backward facing blade, the better its efficiency.
Using your customer’s input data with θᵢ = 46.109° and θₒ = 8° the efficiency increases from 47.5% to 72% and the power required to run the fan drops from 4.4W to 2W. Whilst you lose head and pressure the efficiency gain is greater than the loss.
By setting the outlet angle (θₒ) to 25° you achieve virtually the same head and pressure but with an efficiency of over 57% and a drop in power consumption of 25%

# Note: the efficiency quoted (ε {%}) is for blade design only

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